Image sensor with in-pixel background subtraction and motion detection

ABSTRACT

An imaging system includes a pixel array configured to generate image charge voltage signals in response to incident light received from an external scene. An infrared illumination source is deactivated during the capture of a first image of the external scene and activated during the capture of a second image of the external scene. An array of sample and hold circuits is coupled to the pixel array. Each sample and hold circuit is coupled to a respective pixel of the pixel array and includes first and second capacitors to store first and second image charge voltage signals of the captured first and second images, respectively. A column voltage domain differential amplifier is coupled to the first and second capacitors to determine a difference between the first and second image charge voltage signals to identify an object in a foreground of the external scene.

BACKGROUND INFORMATION Field of the Disclosure

This disclosure relates generally to image sensors, and in particularbut not exclusively, relates to an image sensor for monitoring anexternal scene.

Background

Image sensors have become ubiquitous and are now widely used in digitalcameras, cellular phones, security cameras, as well as medical,automobile, and other applications. As image sensors are integrated intoa broader range of electronic devices, it is desirable to enhance theirfunctionality, performance metrics, and the like in as many ways aspossible (e.g., resolution, power consumption, dynamic range, etc.)through both device architecture design as well as image acquisitionprocessing.

A typical image sensor operates in response to image light from anexternal scene being incident upon the image sensor. The image sensorincludes an array of pixels having photosensitive elements (e.g.,photodiodes) that absorb a portion of the incident image light andgenerate image charge upon absorption of the image light. The imagecharge photogenerated by the pixels may be measured as analog outputimage signals on column bitlines that vary as a function of the incidentimage light. In other words, the amount of image charge generated isproportional to the intensity of the image light, which is read out asanalog image signals from the column bitlines and converted to digitalvalues to provide information that is representative of the externalscene.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 illustrates one example of an imaging system including a voltagedomain global shutter image sensor with an infrared illumination sourceto enhance signal detection of an object of interest in the foregroundof an external scene in accordance with the teachings of the presentinvention.

FIG. 2 illustrates a schematic that shows an example of a pixel cellcoupled to sample and hold circuit and a full column differentialamplifier included in a voltage domain global shutter image sensor inaccordance with the teachings of the present invention.

FIG. 3 is a flow diagram illustrating an example process to detect anobject in the foreground and/or detect motion of the object in anexternal scene with an example voltage domain global shutter imagesensor with an infrared illumination source in accordance with theteachings of the present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. In addition, common butwell-understood elements that are useful or necessary in a commerciallyfeasible embodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

Various examples of an imaging system including a voltage domain globalshutter image sensor with an infrared illumination source to enhancesignal detection of an object of interest in the foreground of anexternal scene are described herein. In the following description,numerous specific details are set forth to provide a thoroughunderstanding of the examples. One skilled in the relevant art willrecognize, however, that the techniques described herein can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail in order to avoid obscuring certain aspects.

Reference throughout this specification to “one example” or “oneembodiment” means that a particular feature, structure, orcharacteristic described in connection with the example is included inat least one example of the present invention. Thus, the appearances ofthe phrases “in one example” or “in one embodiment” in various placesthroughout this specification are not necessarily all referring to thesame example. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner in one or moreexamples.

Spatially relative terms, such as “beneath,” “below,” “over,” “under,”“above,” “upper,” “top,” “bottom,” “left,” “right,” “center,” “middle,”and the like, may be used herein for ease of description to describe oneelement or feature's relationship relative to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is rotated or turned over, elements described as “below” or“beneath” or “under” other elements or features would then be oriented“above” the other elements or features. Thus, the exemplary terms“below” and “under” can encompass both an orientation of above andbelow. The device may be otherwise oriented (rotated ninety degrees orat other orientations) and the spatially relative descriptors usedherein are interpreted accordingly. In addition, it will also beunderstood that when an element is referred to as being “between” twoother elements, it can be the only element between the two otherelements, or one or more intervening elements may also be present.

Throughout this specification, several terms of art are used. Theseterms are to take on their ordinary meaning in the art from which theycome, unless specifically defined herein or the context of their usewould clearly suggest otherwise. It should be noted that element namesand symbols may be used interchangeably through this document (e.g., Sivs. silicon); however, both have identical meaning.

As will be discussed in greater detail below, an example imaging systemin accordance with the teachings of the present invention includes avoltage domain global shutter sensor that utilizes an illuminationsource every other image capture to enhance detection of an object inthe foreground of an external scene. In the various examples, a firstimage of the external scene is captured without any illumination fromthe illumination source, and then a subsequent second image of theexternal scene is captured with illumination from the illuminationsource in sequence. In the examples, the illumination from theillumination source is configured to substantially illuminate an objectin the foreground of the external scene while the background issubstantially not illuminated in the second image. As a result, thefirst image is subtracted from the second image to determine thedifferences between the first image and the second image. The resultingfinal output from the subtraction distinguishes the differences betweenthe first image and the second image, which can be used to identify anobject in the foreground of the external scene and/or identify anymotion that has occurred in the external scene between the first andsecond image captures in accordance with the teachings of the presentinvention.

In various examples, the illumination source is implemented with a lightemitting diode (LED) infrared (IR) illumination source. In one example,the light produced by the LED IR illumination source has a wavelengthsubstantially equal to 940 nanometers, which is not visible to the humaneye. As such, an imaging system in accordance with the teachings of thepresent invention is useful in a variety of applications such as forexample a monitoring system utilized as a vehicle camera to monitor adriver of the vehicle for facial status, eyelid motion, etc. Since theIR light generated by the illumination source is not visible to thedriver, the imaging system is capable of constantly monitoring thedriver without distracting the driver. Other applications of an imagingsystem in accordance with the teachings of the present invention mayinclude, but are not limited to, augmented reality (AR) applications,virtual reality (VR) applications, etc.

To illustrate, FIG. 1 illustrates one example of an imaging system 100including a voltage domain global shutter image sensor with an infraredillumination source to enhance signal detection of an object of interestin the foreground of an external scene in accordance with the teachingsof the present invention. As shown in the example depicted in FIG. 1,imaging system 100 is implemented as a complementary metal oxidesemiconductor (CMOS) image sensor (CIS) in a stacked chipped scheme thatincludes a pixel die 114 stacked with a logic pixel die or applicationspecific integrated circuit (ASIC) die 118. In the example, the pixeldie 114 includes a pixel array 102, and the logic pixel die 116 includesan array of sample and hold circuits 118 that are coupled to the pixelarray 102 through pixel level hybrid bonds 106. Logic pixel die 130 alsoincludes a control circuit 110, a readout circuit 108, and functionlogic 112.

In one example, pixel array 102 is a two-dimensional (2D) array ofphotodiodes, or image sensor pixel cells 104 (e.g., pixel cells P1, P2 .. . , Pn). As illustrated, photodiodes are arranged into rows (e.g.,rows R1 to Ry) and columns (e.g., column C1 to Cx) to acquire image dataof a person, place, object, driver, scene, etc., which can then be usedto render a 2D image of the person, place, object, driver, scene, etc.It is appreciated, however, that the photodiodes do not have to bearranged into rows and columns and may also take other configurations inaccordance with the teachings of the present invention.

As shown in the depicted example, the logic pixel die 116 is stackedwith and coupled to the pixel die 114 in a stacked chip scheme. In theexample, the logic pixel die 116 includes an array of sample and holdcircuits 118 coupled to the readout circuit 108. In the example, eachone of the sample and hold circuits included in the array of sample andhold circuits 118 is coupled to a corresponding one of the pixel cells104 of the pixel array 102 in the pixel die 114 through a respectivepixel level hybrid bond 106 at an interface between the pixel die 114and the logic pixel die 116, which provides a voltage domain globalshutter image sensor in accordance with the teachings of the presentinvention. In particular, each one of the sample and hold circuitsincluded in the array of sample and hold circuits 118 includes first andsecond capacitors configured to store pixel data of the first image andthe second image, respectively, in the voltage domain.

As will be described in greater detail below, a full column voltagedomain differential amplifier is coupled to the sample and hold circuitsof each column of the array of sample and hold circuits 118 to subtractthe first image pixel data from the second image pixel data to determinethe differences between the first image and the second image for eachrow of the array of sample and hold circuits 118. The resulting finaloutput from the subtraction distinguishes the differences between theunilluminated first image and the illuminated second image, which can beused to identify an object in the foreground of the external sceneand/or identify any motion that has occurred in the external scenebetween the first and second image captures in accordance with theteachings of the present invention.

The readout circuit 108 may be used to readout the first and secondimage data and the resulting differences between the first and secondimage data, which may then be transferred to function logic 112. In oneexample, the full column voltage domain differential amplifier may beincluded in the readout circuit 108. In various examples, readoutcircuitry 108 may also include amplification circuitry, analog todigital (ADC) conversion circuitry, or otherwise. In one example,function logic 112 may simply store the image data or even manipulatethe image data by applying post image processing effects (e.g., crop,rotate, remove red eye, adjust brightness, adjust contrast, orotherwise).

In one example, control circuit 110 is coupled to the pixel array 102,the sample and hold circuit array 118, the readout circuit 108, and theinfrared illumination source 120 to control and synchronize theoperation of the pixel array 102, the sample and hold circuit array 118,the readout circuit 108, and the infrared illumination source 120. Inone example, the control circuit 110 is configured to have the imagingsystem 100 capture a first image of an external scene with the infraredillumination source 120 deactivated. In the example, the image data ofthe first image capture from all of the pixel cells 104 of pixel array102 are then globally and simultaneously captured and stored in thevoltage domain in respective first capacitors in the sample and holdcircuit array 118. After the first image is captured without anyillumination from the infrared illumination source 120, the infraredillumination source 120 is then activated to illuminate the foregroundof the external scene and a second image of the illuminated externalscene is then captured. In the example, the image data of the secondimage capture from all of the pixel cells 104 of pixel array 102 arethen globally and simultaneously captured and stored in the voltagedomain in respective second capacitors in the sample and hold circuitarray 118.

In one example, the control circuit 110 is configured to generate anillumination signal ILLUM_SIG 174 that is coupled to be received by theinfrared illumination source 120 to control the infrared illuminationsource 120. In one example, the infrared illumination source 120 isconfigured to direct infrared pulses having a pulse width ofapproximately ˜10 microseconds at a wavelength substantially equal to940 nanometers to the external scene that is being captured by theimaging system 100. It is appreciated that ambient sunlight in theexternal scene happens to have a relatively weak spectrum near 940nanometers at sea level. As a result, the sunlight will have a reducedimpact or effect on the external scene at the 940 nanometer wavelengthcompared to the infrared illumination source 120.

In one example, the data rate of the imaging system is 60 frames persecond, where each frame includes the first and second image captures,with the first image capture being unilluminated by the infraredillumination source 120, and the second image capture being illuminatedby the infrared illumination source 120. In the various examples, thecontrol circuit 110 is configured to regulate the wavelength and powerof the infrared light emitted from the infrared illumination source 120to control the overall heat generated by the infrared illuminationsource 120 that is directed at the objects in the external scene.

FIG. 2 illustrates a schematic that shows an example of a pixel cell 204coupled to sample and hold circuit 218 and a full column differentialamplifier included in a voltage domain global shutter image sensor of animaging system in accordance with the teachings of the presentinvention. It is noted that pixel cell 204 and sample and hold circuit218 of FIG. 2 may be examples of one of the pixel cells 104 and one ofthe sample and hold circuits of the sample and hold circuit array 118described in FIG. 1, and that similarly named and numbered elementsreferenced below are coupled and function similar to as described above.

The example illustrated in FIG. 2 shows a pixel die 214, which isstacked with a logic pixel die 218 as described in FIG. 1. In theexample, the pixel die 214 includes a pixel array that includes pixelcell 204, and the logic pixel die 216 includes an array of sample andhold circuits that includes sample and hold circuit 218, which iscoupled to pixel cell 204 through a respective pixel level hybrid bond206 at an interface between pixel die 214 and logic pixel die 216 asshown.

As shown in the depicted example, pixel cell 204 includes a photodiode222, which is coupled to photogenerate image charge in response toincident light. In one example, the light incident on photodiode 222 maybe ambient light only from an external scene without any illuminationfrom the infrared illumination source 120 during the first imagecapture, or the light incident on photodiode 222 may include infraredlight reflected from the external scene from the infrared illuminationsource 120 during the second image capture.

A transfer gate 224 is coupled to transfer the photogenerated imagecharge from the photodiode 222 to a floating diffusion 226 in responseto a transfer signal TX. A reset transistor 228 is coupled to a supplyvoltage to reset the floating diffusion 226, and the photodiode 222through transfer gate 224, in response to a reset signal RST. The gateof a source follower transistor 230 is coupled to convert the imagecharge in the floating diffusion 226 from the charge domain to an imagecharge voltage signal in the voltage domain, which is coupled to beoutput through the pixel level hybrid bond 206 from pixel die 214 to therespective sample and hold circuit 218 on the logic pixel die 216.

It is noted that in the voltage domain global shutter exampleillustrated in FIG. 2, pixel cell 204 does not include a row selecttransistor coupled to the source follower transistor 230. As such, inthe example depicted in FIG. 2, the drain of the source followertransistor 230 is coupled to the supply voltage through a firstunswitched connection, and the source of the source follower transistor230 is coupled to the pixel level hybrid bond 206 through a secondunswitched connection.

Continuing with the depicted example, the sample and hold circuit 218includes a first sample and hold transistor 236 that is coupled to thepixel level hybrid bond 206 and is configured to sample and hold inresponse to a sample and hold control signal SH1 a first image chargevoltage signal of a first image capture from pixel cell 204 into a firstcapacitor Cbkg 238, which is coupled between first sample and holdtransistor 236 and a low supply voltage DOVDD. In the example, the lowsupply voltage DOVDD is lower in value than the supply voltage, which isconfigured to power the sample and hold circuit 218. In one example, thelow supply voltage DOVDD may be coupled to ground. In addition, thesample and hold circuit 218 also includes a second sample and holdtransistor 244 that is coupled to the pixel level hybrid bond 206 and isconfigured to sample and hold in response to a sample and hold controlsignal SH2 a second image charge voltage signal of a second imagecapture from pixel cell 204 into a second capacitor Csig 246, which iscoupled between second sample and hold transistor 244 and the low supplyvoltage DOVDD. In one example, the first capacitor Cbkg 238 and thesecond capacitor Csig 246 each have a capacitance value equal toapproximately 130 femtofarads.

In the depicted example, the sample and hold circuit 218 includes acurrent source implemented with a transistor 234 that is biased with abias voltage Vbias and is coupled between the pixel level hybrid bond206 and ground. In one example, the sample and hold circuit 218 alsoincludes a reference transistor coupled between the pixel level hybridbond 206 and a reference voltage Vref. In the example, the referencetransistor is configured to couple the reference voltage Vref to thepixel level hybrid bond 206 in response to a reference voltage controlsignal Vctrl.

The example depicted in FIG. 3 shows that the sample and hold circuit218 also includes a first source follower transistor 240 that has a gatecoupled to the first capacitor Cbkg 238 to drive a voltage Vbkg inresponse to the first image charge voltage signal stored in the firstcapacitor Cbkg 238. The voltage Vbkg driven by the first source followertransistor 240 is output through a first row select transistor 242 inresponse to a row select signal RS that is coupled to be received at afirst input of column voltage domain differential amplifier 252. Inaddition, sample and hold circuit 218 also includes a second sourcefollower transistor 248 that has a gate coupled to the second capacitorCsig 246 to drive a voltage Vsig in response to the second image chargevoltage signal stored in the second capacitor Csig 246. The voltage Vsigdriven by the second source follower transistor 248 is output through asecond row select transistor 250 in response to the row select signal RSthat is coupled to be received at a second input of column voltagedomain differential amplifier 252.

In the depicted example, the column voltage domain differentialamplifier 252 is a full column voltage domain differential amplifierthat is coupled to each sample and hold circuit 218 that is included ina column of the sample and hold circuit array 118. In operation, thecolumn voltage domain differential amplifier 252 is configured to outputa difference between the first image charge voltage signal and thesecond image charge voltage signal by subtracting the Vbkg voltage fromthe Vsig voltage. In other words, the output of the column voltagedomain differential amplifier 252 is Vsig-Vbkg. As such, the imagesensor is configured to identify an object in the foreground of theexternal scene that is illuminated by the infrared illumination source120 in response to the detected differences between the first and secondcaptured images determined by subtracting the first image from thesecond image in accordance with the teachings of the present invention.In addition, the image sensor is configured to identify motion in theexternal scene that is illuminated by the infrared illumination source120 in response to the detected differences between the first and secondcaptured images determined by subtracting the first image from thesecond image in accordance with the teachings of the present invention.

FIG. 3 is a flow diagram illustrating an example process 354 to detectan object in the foreground and/or detect motion of the object in anexternal scene with an example voltage domain global shutter imagesensor and an infrared illumination source in accordance with theteachings of the present invention. It is noted that process 354 of FIG.3 refers to processing steps that may be performed by examples of thepixel cells 204 and sample and hold circuits 218 of FIG. 2, or pixelcells 104 and sample and hold circuits included in sample and holdcircuit array 118 of described in FIG. 1, and that similarly namedelements referenced below are coupled and function similar to asdescribed above.

As shown in the example depicted in FIG. 3, processing beings in processblock 356 by deactivating the infrared illumination source. As a result,the external scene is not illuminated by the infrared illuminationsource such that the external scene is illuminated with ambient light.

Process block 358 shows that a first image is then captured with avoltage domain global shutter image sensor as described above withoutany illumination from the infrared illumination source. It isappreciated that this first image capture is an image capture of thebackground of the external scene.

Process block 360 shows that pixel values of the first image are savedin the voltage domain on first capacitors. In the examples describedabove, the first image pixel values may be converted from the imagecharge that is generated by photodiode 222 and saved in the floatingdiffusion 226 in the charge domain into the voltage domain with thepixel source follower transistor 230. The converted first image pixelvalue may then be stored in the first capacitor Cbkg 238 in the voltagedomain.

Process block 362 shows that the infrared illumination source is thenactivated, which in one example is configured to illuminate objects inthe foreground of the external scene with infrared light. In oneexample, the infrared light used to illuminate the foreground objects inthe external scene has a wavelength of approximately 940 nanometers andis therefore not visible to the human eye. In one example, the infraredlight is directed to the foreground objects in the external scene withinfrared pulses having a pulse width of approximately ˜10 microseconds.

Process block 364 shows that a second image is then captured with thevoltage domain global shutter image sensor as described above withillumination from the infrared illumination source. It is appreciatedthat this second image capture is an image capture of the external scenewith the foreground objects illuminated with infrared light from theillumination source.

Process block 366 shows that second image pixel values are saved in thevoltage domain on second capacitors. As in the examples described above,the second image pixel values may be converted from the image chargethat is generated by photodiode 222 and saved in the floating diffusion226 in the charge domain into the voltage domain with the pixel sourcefollower transistor 230. The converted second image pixel value may thenbe stored in the second capacitor Csig 246 in the voltage domain.

Process block 368 shows that the differences between the first capturedimage and the second captured image may be determined by subtracting thefirst captured image pixel values from the second captured image pixelvalues stored in the first and second capacitors in the voltage domain.

Process block 370 shows that a foreground object in external scene offirst and second images is then detected in response to the subtractionof first image pixel values from second image pixel values in voltagedomain as performed in process block 368.

Process block 372 shows that motion in external scene of first andsecond images is then detected in response to subtraction of first imagepixel values from second image pixel values in voltage domain asperformed in process block 368.

The above description of illustrated examples of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific examples of the invention are described herein forillustrative purposes, various modifications are possible within thescope of the invention, as those skilled in the relevant art willrecognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific examples disclosedin the specification. Rather, the scope of the invention is to bedetermined entirely by the following claims, which are to be construedin accordance with established doctrines of claim interpretation.

What is claimed is:
 1. An imaging system, comprising: an array of pixelsconfigured to generate image charge voltage signals in response toincident light received from an external scene; a control circuitcoupled to the array of pixels to control operation of the array ofpixels, wherein the control circuit is configured to control the arrayof pixels to capture a first image of the external scene and a secondimage of the external scene in sequence; an infrared illumination sourceconfigured to illuminate the external scene in response to the controlcircuit, wherein the control circuit is configured to deactivate theinfrared illumination source during the capture of the first image ofthe external scene, wherein the control circuit is configured toactivate the infrared illumination source during the capture of thesecond image of the external scene; an array of sample and hold circuitscoupled to the array of pixels, wherein each one of the sample and holdcircuits is coupled to a respective one of the pixels of the array ofpixels, wherein each one of the sample and hold circuits comprises: afirst capacitor configured to store a first image charge voltage signalof the first image; and a second capacitor configured to store a secondimage charge voltage signal of the second image; and a column voltagedomain differential amplifier coupled to the first capacitor and thesecond capacitor of each one of the sample and hold circuits in a columnof the array of sample and hold circuits, wherein the column voltagedomain differential amplifier is configured to determine a differencebetween the second image charge voltage signal and the first imagecharge voltage signal stored in each one of the sample and hold circuitsto identify an object in a foreground of the external scene.
 2. Theimaging system of claim 1, wherein the column voltage domaindifferential amplifier is further configured to detection motion in theexternal scene in response to the difference between the second imagecharge voltage signal and the first image charge voltage signal storedin each one of the sample and hold circuits.
 3. The imaging system ofclaim 1, wherein the infrared illumination source configured to generateinfrared light having a wavelength substantially equal to 940nanometers.
 4. The imaging system of claim 3, wherein the infraredillumination source configured to generate infrared light pulses havinga pulse width substantially equal to 10 microseconds.
 5. The imagingsystem of claim 4, wherein the imaging system is configured to capturethe first image of the external scene, capture the second image of theexternal scene, and determine the difference between the second imagecharge voltage signal and the first image charge voltage signal storedin each one of the sample and hold circuits 60 times per second.
 6. Theimaging system of claim 1, wherein the first and second capacitors havecapacitance values equal to approximately 130 femtofarads.
 7. Theimaging system of claim 1, further comprising: a first semiconductor diecomprising the array of pixels; and a second semiconductor die stackedwith and coupled to the first semiconductor die, wherein the secondsemiconductor die comprises the array of sample and hold circuits andthe readout circuit.
 8. The imaging system of claim 7, wherein each oneof the pixels of the array of pixels comprises: a photodiode configuredto generate image charge in response to the incident light received fromthe external scene; a floating diffusion configured to store the imagecharge generated by the photodiode; a transfer gate coupled between thephotodiode and the floating diffusion; a reset transistor coupledbetween the floating diffusion and a supply voltage; and a pixel sourcefollower transistor having a gate coupled to the floating diffusion, adrain coupled to the supply voltage, and a source coupled to a hybridbond at an interface between the first semiconductor die and the secondsemiconductor die.
 9. The imaging system of claim 8, wherein the drainof the pixel source follower transistor is coupled to the supply voltagethrough a first unswitched connection, wherein the source of the pixelsource follower transistor is coupled to the hybrid bond through asecond unswitched connection.
 10. The imaging system of claim 8, whereineach one of the sample and hold circuit further comprises: a firstsample and hold transistor coupled between the hybrid bond and the firstcapacitor; a first source follower transistor having a gate coupled tothe first capacitor; a first row select transistor coupled between thefirst source follower transistor and a first input of the column voltagedomain differential amplifier; a second sample and hold transistorcoupled between the hybrid bond and the second capacitor; a secondsource follower transistor having a gate coupled to the secondcapacitor; and a second row select transistor coupled between the firstsource follower transistor and a first input of the column voltagedomain differential amplifier.
 11. The imaging system of claim 10,wherein each one of the sample and hold circuit further comprises acurrent source coupled between the hybrid bond and ground, wherein thecurrent source comprises a current source transistor having a gatecoupled to receive a bias voltage.
 12. The imaging system of claim 11,wherein each one of the sample and hold circuit further comprises areference transistor coupled between the hybrid bond and a referencevoltage, wherein the reference transistor is configured to provide thereference voltage to the hybrid bond in response to a reference voltagecontrol signal.
 13. The imaging system of claim 1, wherein the columnvoltage domain differential amplifier is included in a readout circuitcoupled to receive image charge voltage signals of the first image fromthe array of pixels and image charge voltage signals of the secondimage.
 14. A method for monitoring an external scene, comprising:deactivating an infrared illumination source; capturing a first image ofthe external scene with a global shutter image sensor withoutillumination from the infrared illumination source; saving first imagepixel values of the first image in a voltage domain on first capacitors;activating the infrared illumination source; capturing a second image ofthe external scene with the global shutter image sensor withillumination from the infrared illumination source; saving second imagepixel values of the second image in the voltage domain on secondcapacitors; detecting an object in a foreground of the external scene inresponse to subtracting the first image pixels values of the first imagefrom the second image pixels values of the second image in the voltagedomain.
 15. The method of claim 14, further comprising detecting motionin the external scene in response to said subtracting the first imagepixels values of the first image from the second image pixels values ofthe second image in the voltage domain.
 16. The method of claim 14,wherein said activating the infrared illumination source occurs aftersaid capturing the first image of the external scene with the globalshutter image sensor without illumination from the infrared illuminationsource and during said capturing the second image of the external scenewith the global shutter image sensor with illumination from the infraredillumination source.
 17. The method of claim 14, wherein said activatingthe infrared illumination source comprises directing light having awavelength substantially equal to 940 nanometers from the infraredillumination source to the external scene.
 18. The method of claim 14,wherein said activating the infrared illumination source comprisesgenerating infrared pulses of light having a pulse width substantiallyequal to 10 microseconds.
 19. The method of claim 14, wherein saidcapturing the first image of the external scene with the global shutterimage sensor without illumination from the infrared illumination source,said capturing the second image of the external scene with the globalshutter image sensor with illumination from the infrared illuminationsource, and said detecting the object in the foreground of the externalscene in response to subtracting the first image pixels values of thefirst image from the second image pixels values of the second image inthe voltage domain occur 60 times per second.
 20. The method of claim14, wherein the first and second capacitors have capacitance valuesequal to approximately 130 femtofarads.